Need to find the net force and the average force....

A bullet of mass 12 g strikes a stationary fixed block of wood at a speed of 400m/s, penetrating to a depth of 3.0cm. Calculate the average net force acting on the bullet while it is in the wood. Calculate the average force exerted on the wood by the bullet.

2. Relevant equations
Fnet= m x a
W=1/2mv^2

3. The attempt at a solution
I found the force net using the kinetic energy and work equation. I don't really know how to find the average force.

Try using this equation: 2ax = (vf)^2 - (vi)^2.
Find the acceleration.
Use Newton's second law to calculate force.
This is the only possible solution I can see.

Note: use above equation only if a is constant.
I like your approach using work-kinetic energy theorem.
One point: the force exerted on the wood by the bullet isn't the same force exerted by the wood on the bullet with opposite direction? (3rd Newton's Law)

Fnet is equal to 79.998 N. for part b it wants me to find force average. when i think of force average I think of momentum. but i don't have enough information to find force average using momentum. Do you know any other equations???

Mmmm... I have just tried to solve this problem using the equation of kinematics I gave and the work-kinetic energy theorem and both gave me that the average force is 32000 N. Do you know what is the correct answer (from the textbook, if you took the problem from one)?

About using momentum, you must have a function that describes how it changes over time, in order to derivate it and find force. Then, you will need to integrate the force and divide it by the time interval. Unfortunately, I see no way of doing this for this kind of exercise.